The present invention relates to a method of controlling intercellular handover (HO) in a multicellular radio communications network, as well as to methods for estimating speeds of movement of mobile stations, which can be used in implementing such handover control method.
Mobile radio communications systems mainly comprise the equipments of the mobile telecommunications service and the mobile stations. The equipments of the mobile telecommunications service generally consist of two subassemblies: the message routing and management system on the one hand, and the radio system on the other hand.
The message routing and management system has as its main functions: interconnection of the radio communications system with a static network (for example the public switched telephone network), recognition and analysis of connection requests from static or mobile subscribers, the search for static or mobile correspondents, management of resources in terms of radio channels and of links with the static users, routing of messages from the user to the radio channel and supervision of communications. To do that the system comprises switches, computers and databases (which, in particular, store the information relating to the subscribers).
The radio system undertakes the radio transmission of messages between the parties. Its main functions are therefore message transmission by means of radio, supervision of the continuity of the links and protection against third parties. The radio system consists of radio relays called base stations, which may be static or mobile (satellites for example).
In order to optimize the use of the radio spectrum, in particular in zones with high subscriber density, a partitioning into cells, based, for example, on the technique of frequency reuse or of dynamic channel allocation, has been adopted in the majority of systems. For that reason these systems are called cellular networks.
One of the important functions of the message routing and management system consists in ensuring the continuity of the communication when a terminal is moving. The mobile radio communications system must, in fact, avoid the link being cut between the terminal and the static network, particularly when this link is supporting a communication. Cut off can occur in particular when the mobile crosses the boundary of the cell to which it is attached. It is then a question of allowing handover of the link from the base station left behind to a new base station which will be able to serve the mobile satisfactorily (that is to say which will make it possible to provide the link established with the characteristics of the service or services required). From a general point of view, HO ("handover" or "handoff") consists in changing the physical channels (radio channel and/or channel of the support network which are associated with the service connection) which are necessary to maintain the communication.
In zones with high subscriber density, small-size cells are formed (called microcells or picocells). These cells are used to serve low-speed or static mobiles, that is to say mainly pedestrians. As far as higher-speed mobiles are concerned, for example motor cars, the times needed to execute a HO between microcells or picocells may be too short for these mobiles to be able to be connected thereto (such cells may constitute one or two layers of cells). It is for this reason that a network of more extensive cells, called macrocells or umbrella cells, intended to serve mobiles of relatively "high" speed, is superimposed on these small-size cells. This network may also serve as a backup to the network of small-size cells.
Moreover, the high speed of some trains as well as the appearance of mobiles in aircrafts require that the HO algorithm has to usable in networks with several layers (&gt;2). It is indeed possible to imagine a mobile in a train, with zero speed of movement in a station, low speed on leaving the station and high speed when the train is at full speed.
A network consisting of several layers of cells is called a "multilayer" network, "multicellular" network or also "microcellular" network, this latter term being reserved for the case of one layer of microcells and one layer of macrocells.
In "conventional" mobile networks, that is to say networks consisting of a single layer of cells the radius of which is generally a few kilometers (typically 1 km to 30 km), the HO algorithms are based mainly on criteria of field level, of signal quality (for the digital systems), of mobile-base station distance and of signal attenuation (comparison of the attenuation of signals originating from several base stations).
In general, when a mobile moves away from the base station to which it is attached, the field level which it receives therefrom (as well as that which the base station receives from the mobile) decreases, the number of erroneous bits in the messages exchanged increases as does the distance and the attenuation of the signals sent.
The decrease in the field level does not occur uniformly. The radio frequency signal is in fact subject to three types of variations, average attenuation, slow fading, and rapid fading. Average attenuation is the only one of the three components which is present in an environment free of any obstacle. Slow fading is due to the presence of obstacles to propagation, such as buildings, and depends on the speed of movement of the mobile. It causes a slow variation in the signal about the average field. Rapid fading is due to multiple paths which a signal can take in order to propagate from a transmitter to a receiver due to diffraction or reflection from buildings. It generates a rapid variation in the signal.
When the received field level becomes too weak, the signal quality too poor or the distance too great, the message routing and management system can trigger a HO by seeking the base station which is the most suitable to continue the communication.
A HO can also be triggered, on an attenuation criterion, even before one of the three foregoing criteria is satisfied; to do that, it is sufficient for the attenuation of the field level received from a neighboring base station to be lower, to within a margin, than that of the base station to which the mobile is attached.
In the European GSM system, this type of criterion is called PBGT (Power BudGeT), and the margin HO.sub.-- MARGIN(n1,n2) (n1: source cell, n2: target cell). It ensures that the mobile will be attached to the base station with lowest attenuation. It makes it possible, in particular, to minimize interference in frequency-reuse systems. In the particular case of GSM networks, the measurements made by the mobile are transmitted to the network over the SACCH uplink channel (MEASUREMENT REPORT message) every 480 ms (or every 960 as if the current service is the short-messages service). The measurements taken by the base station (BTS) to which the mobile is attached are added to those received in the MEASUREMENT REPORT message from the mobile, in order to form the MEASUREMENT RESULT message which is sent to the base station controller (BSC). It is on the basis of this information that the BSC can, for example, trigger a HO. The measurements taken and the associated procedures are described in GSM Recommendation 05-08 (draft pr ETS 300 578, 2nd edition, March 1995, European Telecommunications Standards Institute). Annex A to this Recommendation gives a complete example of a HO and power monitoring algorithm.
The document GB-A-2,273,424 describes a HO control method for a single-layer network, comprising an anticipated estimation of the instant at which a mobile station will cross the boundary between two cells. This instant is estimated on the basis of an extrapolation of the DISTANCE parameter included in the MEASUREMENT REPORT message, deduced from the TIMING ADVANCE parameter necessary for TDMA operation. However, the precision in this distance parameter is only of the order of 500 m, so that this method can be used only for source cells of relatively large size. The document further proposes to monitor the profile of the signal level received by the mobile station from the base station of the source cell, in order to inhibit the handover when a rapid mobile station suffers significant masking (typically, a mobile station on board a train passing through a tunnel). However, this assumes a prior knowledge of the trajectory of the mobile, and does not make it possible, in the general case, to distinguish between the slow fading affecting a rapid mobile and the rapid fading affecting a slow mobile.
In the case of a multicellular network, it is sought to assign the "rapid" mobiles to the macrocells and the "slow" mobiles to the microcells. One known solution for taking account of the speed of movement of the mobiles consists in delaying the triggering of the HO. When a handover criterion (PBGT for example) is satisfied, a time delay is triggered. If the mobile is rapid and connected to a macrocell, the PBGT criterion for handover to a microcell will no longer be satisfied on expiry of the time delay, since the mobile will have passed through the microcell. In this case, the mobile does not perform a HO. If the mobile is rapid and is connected to a microcell, it will trigger a HO on field level before the expiry of the time delay. In this case, only one macrocell is allowed to be the candidate. If the mobile is slow and is connected to a macrocell, the PBGT criterion for handover to a microcell will always be satisfied upon expiry of the time delay. In this case, a HO to the microcell will be performed. If the mobile is slow and is connected to a microcell, the PBGT criterion for handover to another microcell will always be satisfied upon expiry of the time delay. In this case, a HO to the target microcell will be performed. If the PBGT criterion for handover is satisfied for a macrocell, then the mobile will trigger a HO to that macrocell.
In three of the four cases mentioned above, it is necessary to wait until expiry of the time delay in order to take a decision concerning the HO. A typical value for this time delay is 40 seconds. The additional movement performed by the mobile during this time may therefore be significant. Moreover, a rapid mobile connected to a microcell will have to wait until a handover criterion on field level is satisfied in order to perform a HO, which also delays the handover.
Due to this wait, the interference generated by frequency reuse is greater than it is with a method which makes it possible to trigger the HO as soon as the handover criterion is satisfied.
One object of the present invention is to propose a method making it possible, in a multicellular network, to trigger intercellular handovers rapidly by taking account of the speed of movement of the mobile stations.